Thermosets, such as epoxy resins, are brittle compared to thermoplastics because of their cross-linked molecular structures. Attempts have been made in the past to increase the toughness of thermosets, particularly epoxies, using the addition of liquid rubber or hard particles.
Liquid rubber has been successfully used as a toughening agent to increase the specific fracture energy. Its toughening mechanisms include bridging, cavitation, crack pinning, crack blunting etc. In addition to liquid rubber, other toughening agents such as hard particles, hard hollow microspheres and coreshell rubber have also been used.
Another development in this area is an attempt to toughen thermoplastics using a similar method to the one used for ceramics in which fracture toughness increase was achieved by a volume dilatation in the vicinity of crack tip resulted from tetragonal to monoclinic phase transformation.
The present invention results from the realisation that the pre-stressing of the epoxy matrix, and the creation of residual compressive stress may be performed using expandable hollow microspheres and heat treatment to achieve a similar effect to that of the phase transformation of ceramics.